Multivibrator



Oct. 21, 1958 INVENTOR. LAURENCE H. DUNKLEE BY I ATTORJ JEY MULTIVIJBRATGR Laurence H. Dunklee, Garden Grove, Califi, assignor to North American Aviation,

Application April 4, 1955, Serial No. 499,054

4' Claims. (Cl. 250-27) This-invention pertains to multivibrators and in a particular application frequency dividers in which a relatively high exciting frequency is applied to a multivibrator and a submultiple of the exciting frequency is obtained at the output. It particularly pertains to a multivibrator frequency divider which is reliable and relatively insensitive to fluctuations in heater current and plate voltage. Particular use for these devices is found in frequency standards.

This invention is an improvement on the multivibrator frequency divider invented by David H. Campbell, filed January 31', 1950, Serial No. 141,360, now Patent No. 2,710,918, issued June'14, 1955. a

In a multivibrator of this type a D.-C. source is used to chargea capacitor to a given level at which it discharges through a path containingone or more resistive elements; Since a combination of resistors and a capacitor has a time constant which may be expressed as the sum of the resistors multiplied by the capacitance of the capacitor, the multivibrator will have a time constant which is dependent to a great extent upon the values of these elements. In frequency dividers employing tubes, at least one of the resistances in series with the capacitor may constitute the resistance between the elements of a vacuum tube. Inasmuch as this resistance depends upon various other parameters, such as plate voltage and filament voltage, the time constant of the multibrator may be undesirably altered by reason of variation in plate or filament voltage.

This invention contemplates a scheme for automatically compensating for such changes to maintain the time constant of the circuit relatively unchanged.

It is therefore an object of this invention to provide an improved multivibrator.

It is another object of this invention to provide an improved multivibrator frequency divider.

It is still another object of this invention to provide a multivibrator frequency divider of improved stability.

It is still another object of this invention to provide a multivibrator frequency divider which is relatively insensitive to changes in plate voltage.

It is still another object of this invention to provide a multivibrator frequency divider having fewer tube failures.

It is still another object of this invention to provide a multivibrator capable of operation over a wide range of frequencies by simple substitution of capacitance.

It is a further object of this invention to provide a multivibrator frequency divider which does not depend on grid to cathode conduction.

Other objects of invention will become apparent from the following description taken in connection with the accompanying single figure.

The signal the frequency of which is to be divided is received at the grid of tube 1 through potentiometer 2 and input capacitor 3. The cathode of tube 1 is connected through resistor 4 to ground. The plate of tube 1 is connected through resistors 5 and 6 to the cathode of tube 7. The cathode of tube 7 is connected through capacitor 8 to the cathode of tube 1. The common connection of resistors 5 and 6 is connected to the grid of tube 7, and the plate of tube 7 is connected through resistor 9 to the B+ and through capacitor 10 to the signal output'terminal 11. 1

To utilize the multivibrator as a frequency divider, it

is first designed to free-run at a frequency slightly below the output frequency desired.

In operation, capacitor 8 first commences to charge through tube 7 and resistors 9 and 4. Resistor 4 being in the cathode circuit of tube 1, prevents tube 1 from conducting during this interval. As the capacitor 8 becomes charged, and the current reduces throughresistors 9 and 4, the drop across resistor 4 is no longer sufiicient" to maintain tube 1 at cutoff At this point, capacitor 8 discharges through the path of resistor 6, resistor 5, and

second,.or third pulse of the input frequency up to a division of approximately ten, whichever is desired. 7,

Without the symmetry in the charge and discharge paths previously. expressed herein, various values of components must be chosen by trial and error to obtain suitable frequencies of operation and division. As a result, the output voltage of tube 7 would likewise be dependent upon the frequency and the values of the parameters chosen. This leads to the difficulty in design of coupling circuits in that the circuit at different frequencies provides different voltage outputs.

A practical arrangement is, of course, to have all resistors equal in value. The free-running frequency then of the multivibrator varies inversely as the capacitance 8 is varied.

It is to be realized, of course, that the resistors 5 and 9 may be decreased in value substantially to little or no resistance. It only being desirable that their values be comparable.

Various circuit components are as follows:

Tubes 1 and 7, 12AU7, resistors 4, 5, 6 and 9 are preferably larger than 2,000 ohms and of a tolerance depending on accuracy desired. Potentiometer 2 may be 5,000 ohms and plate supply voltage, 13+, 50 to 250 volts, It is not advisable to swing the voltage above 200 volts except intermittently- Resistors 4, 5, 6 and 9 may vary in value, for example, from 2,000 ohms to 22,000 ohms. Capacitor '8 may vary in value from approximately 50 rnicromicrofarads to, for example, 1 microfarad. These ranges should provide free-running frequency from just below 1 megacycle to 10 cycles.

It is significant that the number of resistors in the path charging capacitor 8 is equal to the number of resistors in the path discharging capacitor 8, and the resistors are of comparable values. In this way, the conduction period of the tubes becomes equal and with a 50-50 duty cycle the tubes have longer life and the circuit has greater stability. Inasmuch as the paths are similar, a frequency divider having a range of from 10 cycles to 500 kilocycles may be obtained as desired merely by the substitution of capacitor 8 for other capacitors.

It can be seen from the arrangement of tubes 1 and 7 i that if one tube increases in its conduction by reason of increase in plate voltage, or filament voltage, the other one will also increase in conduction by reason of the fact that they are similar in characteristics. It has been found Patented Oct. 21, 1958 that the device of the invention is relatively unchanged in its frequency by large variations in plate voltage.

The output pulse amplitude of this device is nearly constant with frequency and is sufliciently large to insure positive signal to the succeeding stages if they are similar dividers: 'Reliable division down to a ratio of one-tenth is practical. A variation of plus or minus 75 volts in the supply voltage"(it normally being 150 volts) appears to have no efiect on the desired frequency division. It is not r'equisite that resistors 6 and 5 be equal to resistors.

9 and'4; butresistor-5 should be equal to resistor 9 and resistor 4 should be equal to resistor6, in order to acquire a 5050 duty cycle for each tube.

1 Phase shift between the output and the input of this frequencyldivider is negligible;

A further advantage of the equal duty cycle between tubes is that the wave form contains the largest ratio of fundamental signal to harmonics. The equal duty cycle, therefore, produces the largest sinusoidal output after having been fed to a suitable filter.

The signal output may, of course, be obtained at other points in the circuit, as for example, through capacitor 12 at terminal 13. Whereas at terminal 11 the output would be a negative pulse, the output at terminal 13 would be a positive pulse. If desired, these two terminals could be used together, providing a push-pull output.

Although the invention has been described and illustrated in detail,'it is to be clearly understood that the same is by way of illustration and example-only and is not to be taken" by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

- 1. A multivibrator frequency divider comprising a storage capacitor, a D.-C. source, first, second, third, and fourth resistors, a first tube having at least a cathode, an

anode, and a control grid, the anode of said first tube connected to the positive terminal of said D.-C. source through said first resistor, the cathode of said first tube being connected to one terminal of said storage capacitor,

the other terminal of said storage capacitor being connected through said second resistor to the negative terminal of said D.-C. source, a second tube having at least a cathode, an anode, and a control grid, the anode-cathode circuit of said second tube connected through said third I and fourth resistors in series across said capacitor so as to provide a discharge path for said capacitor, the grid of said first tube being connected between said third and fourth resistors, the grid of'said second tube connected to be controlled by a signal input frequency.

2. The combination recited in claim lwherein said first and third resistors are of substantially equal value, and said second and fourth resistors are of substantially equal value.

resistors are of substantially equal value.

4. The combination recited in claim 1 wherein said tubes areof'siinilar characteristics and 'said resistors are all of substantially equal value. g 7

References Cited in the'file of this patent UNTTED STATES PATENTS 3. The combination recited in claim l wherein all said:

Wilson j Nov. 12, 1940 

